This invention relates to a hydraulic control apparatus for an automatic transmission.
An example of a conventional hydraulic control apparatus for controlling an automatic transmission is disclosed in the specification of Japanese Patent Publication (KOKOKU) No. 57-20503. This conventional hydraulic control apparatus is equipped with shift valves the number of which is the same as the number of gear changes made by the automatic transmission. The apparatus is adapted to changeover hydraulic pressure by successively actuating the shift valves one after another for successive speed changes at operating points set in such a manner that governor pressure commensurate with vehicle velocity and throttle commensurate with the amount of accelerator pedal depression act against each other. It is also so arranged that overdrive clutch pressure and overdrive brake pressure are changed over to bring about a fourth speed, namely an overdrive state, from third speed.
In a conventional hydraulic control apparatus of this kind, a changeover between overdrive clutch pressure and overdrive brake pressure can be performed in first or second speed to achieve an intermediate speed, namely a 1.5 or 2.5 speed, respectively, thereby improving the power performance and fuel consumption of the vehicle.
However, when performing an intermediate shift to the 1.5 speed or 2.5 speed as mentioned above, it is necessary to changeover the overdrive pressure whenever such a shift is made. Since the above-described conventional hydraulic control apparatus is so adapted that the operating point of each shift valve is set for a single speed change, control for setting the intermediate speed is difficult in actual practice, even if feasible in terms of mechanical arrangement.